Process for heat-treating titanium in a fused bath



Patented May 22, 19 51 PROGESS FOR HEAT-TREATING TITANIUM IN FUSED BATHRoss M. Mayfield, Elmhurst, Ill., and William B.

De Long, Newark, and Philip H. Permar, Wilmington, Del., assignors, bymesne assignments, to Remington Arms Company, Inc., a corporation ofDelaware No Drawing. Application March 16, 1950,

Serial No. 150,112

This invention relates to the processing of titanium and its alloys, andcontemplates treatment by means of a salt composition adapted to form ontitanium and its alloys a thin adherent protective surface coating. Themolten salt may be used as an annealing bath, a rolling lubricant, orotherwise.

This application is a continuation-in-part of abandoned applicationSerial No. 114,452, filed September 7, 1949, by the same applicants.

By reason of the extraordinary chemical reactivity of titanium,particularly at elevated temperatures, it becomes difficult to maintainthe metal and. its alloys at a temperature and for a time which effectadequate annealing. Atinospheric oxygen and nitrogen are both readilyabsorbed in the surface metal at relatively low temperatures, and athigher temperatures these absorbed gases diffuse into the interior,materially altering tensile properties and hardness. Annealing in avacuum, or in an atmosphere of an inert gas, such as argon, istroublesome and not entirely satisfactory, due to the almost inevitablepresence of fractions of reactive gases in the closed annealing chamber.The common salt baths at the usual working temperature readily attackand destroy titanium.

The present invention comprises the discovery that the presence of anadequate amount of lithium in a conventional salt bath, say onecomprising a halogen salt of an alkali metal, and an oxidizing salt,produces a thin adherent surface film which completely protects themetal from attack. Such a salt bath as one comprising equal parts ofpotassium chloride and sodium carbonate very readily attacks titanium.However, if about 6% of a lithium compound, such as lithium chloride, isadded to this bath, a protectivefilm is formed, which enables theannealing of titanium and its alloys at temperatures at least as high as1600 F. Moreover, the bath may be held at 1600 F. for severalhour's,-a'nd at a temperature of 1200 F. for a period of days, withoutappreciable loss of lithium or other undesired effect. Workingtemperatures, up to about 1600 F., are determined by the resultsdesired. The characteristic of this salt composition is its ability toform a titanate, e. g., lithium titanate, at working temperature, and toaccomplish this purpose it has been found that the bath must containlithium, an oxidizing radical, and a halide, other than a fluoride, ofan alkali metal.

The present invention comprises not only the discovery of thepassivating effect of lithium in 7 Claims. (01. 148--13.1)

molten salt baths which otherwise attack titanium, but further discoverythat lithium is retained in such baths at temperatures up to at least1600 F. Prior uses of lithium salts in salt baths have been merely as analternate for another alkali metal salt, in baths which under operatingconditions did not attack the metal under treatment even with no lithiumpresent; or as a means of lowering the melting point of the salt mixtureto enable its use at lower temperatures. The working temperatures of1200 F. to 1600 F. contemplated under this invention are some hundredsof degrees higher than hitherto proposed for baths containing lithium,and the baths of this invention are the only ones known which do notattack titanium and its alloys at temperatures at which titanium can beannealed.

A preferred bath comprises about 47% potassium chloride, about 47% ofsodium carbonate, and about 6% lithium chloride. This formula, however,is subject to wide variation.

As above set forth, the essential ingredients have been found to belithium, an oxidizer, and a halogen salt of an alkali metal. Presentinthe bath are ions of lithium, ions of one or more of the alkali metals,and ions of a halogen other than'fiuorine. The lithium may be introducedas a metal, or it may be derived from one or more of a wide variety oflithium salts including the fluoride, carbonate, phosphate and bromide,as well as the chloride. If lithium chloride is used, it supplies a partof the halide, the remainder being sodium or potassium chloride, iodideor bromide. Fluorides alone are not satisfactory, although lithiumfluoride may be used as a source of lithium provided an alkali-metalsalt of another halogen is also present. The oxidizer may be ahydroxide, peroxide, chromate, phosphate, molybda'te or vanadate, aswell as a carbonate. As to proportions, the lithium ion content shouldbe not less than 0.? weight percent, which, in a typical bath issupplied by a lithium chloride content of 4.25%. The preferred lithiumion concentration is about 1% (6% Li Cl) but it may be as high as 6.6%(40% Li C1). The concentration of the oxidizing ion should be not lessthan about 0.3 weight percent, and preferably not over about 35 weightpercent, concentrations between 2% and 27% being preferred.

The more commonly used halogen suppliers are potassium and sodiumchloride, and the most convenient oxidizers are potassium and sodiumcarbonate. With 6% weight lithium chloride,

these may be used in all combinations and wide ranges of proportions, asfollows:

Sodium peroxide Weight Weight Weight Weight Per Cent Per Cent Per Cent 1Per Cent Lithium Chloride 6 l 6 30 6-30 1 6 Potassium Chloride -93 -93Potassium Carbonate. 0. 5-84 2 3-69 Sodium Chloride 30 93 25 92 SodiumCarbonate 0. 5-64 1-64 Other efiective baths are as 'follows:

Various halogens and lithium salts:

Sodium carbonate 44. 2 43. 8 46. 3 47 Potassium chloride 47. 6 44. 2 43.8 46. 3

Potassium bromid 45 Potassium iodide 47 Lithium fluoride 4 8 Lithiumcarbonate I. ll. 6

Lithium phosphatal 12.4

Lithium bromide 7. 4 10' Lithium chloride l 6 Various oxidizersPotassium chloride 84 84 84 84 Lithium chloride 6 6 Sodium hydro\ide l0Sodium chromate Sodium phosphate. Sodium vanadatc Sodium molybdatc l. n10 The surface film which forms on titanium and its alloys, whenimmersed in the salt baths abovedescribed, is extremely thin,iridescent, and of a purple-to-black color. It has been found to containtitanium dioxide and lithium titanate, as well as compounds notidentified. Its major constituent is titanium. It also contains a smallpercentage of lithium, and only low traces of other elementsidentifiable by spectrograph analysis. By the use of sodium hydride itis readily removed as desired, for further processing or surfacefinishing of the metal.

What'is claimed is:

1. In the processing of titanium and its alloys, the method whichcomprises treating the metal with a molten salt composition at atemperature between 1200 F. and 1600 F., said composition consistingessentially of ions of lithium in a concentration between 0.7% and 6.6%;ions selected from the group consisting of chloride, iodide and bromide;and oxidizing ions capable of forming a titanate at the workingtemperature.

2. In the processing of titanium and. its alloys.

the method which comprises treating the metal in a molten salt bath,said bath consisting essentially of a halogen salt of lithium in such anamount as to provide a lithium ion concentration of from 07% to 6.6%, a.halogen salt of another alkali metal selected from the group consistingof chlorides, iodides and bromides, and an oxidizingsalt selected fromthe group consisting of theperoxides, hydroxides, carbonates, chromates,phosphates, molybdates and vanadates of sodium and potassium.

4. In the processing of titanium and its alloys, the method whichcomprises treating the metal in a molten salt bath, said bath consistingessentially of a lithium salt in such an amount as to yield aconcentration of lithium ions between about 0.7 and about 6.6 weightpercent, a carbonate in such an amount as to yield a carbonate ionconcentration between about 0.3 and about 36 weight percent, balance analkali-metal salt selected from the group consisting of chlorides,bromides and iodides.

5. In the processing of titanium and its alloys, the method whichcomprises treating the metal in a molten salt bath at a temperaturebetween 1200 F. and 1600 F., said bath consisting essentially of betweenabout 6% and about 30% of lithium chloride, between about 0.5% and about64% of sodium carbonate, and the remainder a chloride selected from thegroup consisting of the chlorides of sodium and potassium.

6. In the processing of titanium and its alloys, the method whichcomprises treating the metal in a molten salt bath at'a temperaturebetween 1200 F. and 1600 F., said bath consisting essentially of betweenabout 6% and about 15% of lithium chloride, 0.5% to about 84% ofpotassium carbonate, and the remainder a chloride selected from thegroup consisting of the chlorides of sodium and potassium.

7. In the processing of titanium and its alloys, the method whichcomprises treating the metal in a molten salt bath at a temperaturebetween 1200" F. and 1600 F., said bath consisting essentially of'about47% sodium carbonate, about 47% potassium chloride, and about 6% lithiumchloride.

ROSS M. MAYFIELD. WILLIAM B. DE LONG. PHILIP I-I. PERMAR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED. STATES PATENTS Number Name Date" 2,066,454 Bonath et a1. Jan. 5,1937 2,148,664 Wille Feb. 28, 1939 FOREIGN PATENTS Number Country Date114,324 Sweden Apr. 26, 1945

1. IN A PROCESSING OF TITANIUM AND ITS ALLOYS, THE METHOD WHICHCOMPRISES TREATING THE METAL WITH A MOLTEN SALT COMPOSITION AT ATEMPERATURE BETWEEN 1200* F. AND 1600* F., SAID COMPOSITION CONSISTINGESSENTIALLY OF IONS OF LITHIUM IN A CONCENTRATION BETWEEN 0.7% AND 6.6%;IONS SELECTED FROM THE GROUP CONSISTING OF CHLORIDE, IODIDE AND BROMIDE;AND OXIDIZING IONS CAPABLE OF FORMING A TITANATE AT THE WORKINGTEMPERATURE.